This invention generally relates to integrated circuitry for use in conjunction with ultrasound transducer elements. In particular, the invention relates to ultrasound transmit application-specific integrated circuits (ASICs) for use in ultrasound imaging systems.
A medical ultrasound imaging system forms an image by acquiring individual ultrasound lines (or beams). The lines are adjacent to each other and cover the target area to be imaged. Each line is formed by transmitting an ultrasonic pulse in a particular spatial direction and receiving the reflected echoes from that direction. The spatial characteristics of the transmitted wave and the characteristics of the receive sensitivity determine the quality of the ultrasound image. It is desirable that the ultrasound line gathers target information only from the intended direction and ignores targets at other directions.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements that are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves that combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
The quality or resolution of the image formed is partly a function of the number of transducer elements that respectively constitute the transmit and receive apertures of the transducer array. Accordingly, to achieve high image quality, a large number of transducer elements is desirable for both two- and three-dimensional imaging applications. The ultrasound transducer elements are typically located in a hand-held transducer probe that is connected by a flexible cable to an electronics unit that processes the transducer signals and generates ultrasound images. The transducer probe may carry both ultrasound transmit circuitry and ultrasound receive circuitry.
It is known to include high-voltage components in the transmit circuitry to drive the individual ultrasound transducer elements, while low-voltage, high-density digital logic circuitry is used to provide transmit signals to the high-voltage drivers. The high-voltage drivers typically operate at voltages of up to approximately 100 volts, while the low-voltage logic circuitry has an operating voltage on the order of 5 volts in the case of TTL logic. The high-voltage drivers may be fabricated as discrete components or as integrated circuits, while the low-voltage logic circuitry may be fabricated as a separate integrated circuit or combined with the high-voltage circuitry on a single chip. In addition to transmit circuitry including the high-voltage drivers and low-voltage logic circuitry, the transducer head may include low-noise, low-voltage analog receive circuitry. The low-voltage receive circuitry, like the transmit logic circuitry, typically has an operating voltage on the order of 5 volts, and may be a separate integrated circuit or may be fabricated with the low-voltage transmit logic circuitry as a monolithic integrated circuit.
In order to maximize the number of transducer elements to achieve high-quality ultrasound images, it is desirable to integrate as much circuitry as possible in as small a volume as possible to reduce the size and complexity of the circuitry, whether the circuitry be located within a transducer probe or in an electronics unit separate therefrom. In addition, some applications, for example, very high-frequency ultrasound imaging, require that transmit circuitry be located as close as possible to the transducer elements to avoid signal loading by a long cable. Therefore it has been proposed to integrate the aforementioned high-voltage drivers with either or both of the low-voltage transmit logic circuitry and the low-voltage receive circuitry as a monolithic integrated circuit.
In typical high-voltage integrated circuit processes, variation of the transistor drain-source current with processing can be as much as 1:2. This leads to significant variation in the rise time of a high-voltage ultrasound transmitter stage. This variation can yield distortions in the ultrasound image. The standard way to mitigate this phenomenon is to design transistors that are twice as large as normal (twice the drain-source current), and trim off until the desired drain-source current is attained. This leads to increased cost due to the need for trimming and larger die area.
In ultrasound imaging, especially when used for echo-cardiography, second harmonic distortion leads to a decrease in sensitivity of the system. Such distortion can be quantified by taking the Fourier spectrum of the transmitted signal and measuring the magnitude of the peak that is the second harmonic above the fundamental or transmit frequency.
Lack of symmetry between rise and fall times of the pulse edges is one source of second harmonic distortion. This effect can be mitigated by carefully matching the transistors used in the output stage of the transmitter. Such matching is typically done either through trimming or by sorting of discrete devices in standard manufacturing practice. When using a monolithic transmitter however, sorting of devices becomes more costly.
There is a need for improved methods and devices for controlling the rise and fall times of the output pulse in an ultrasound transmitter integrated circuit.